Conventional polarizing elements include polarizing beam splitters that use a dielectric multilayer film, Rochon prisms and Glan-Thompson prisms that use a birefringent crystal, and linearly polarizing films that are manufactured by drawing an organic compound resin film in one direction, thus orienting iodine or a dichroic dye in a specific direction.
Moreover, a polarizing element characterized in that fine silver particles having shape anisotropy are precipitated and dispersed in glass is known (see, for example, New Glass, Vol. 12, No. 4, 1997, p 42). According to this polarizing element, glass in which fine silver halide particles have been dispersed is hot drawn, and as a result the fine silver halide particles are transformed into spheroids, and at the same time these spheroids are oriented such that the major axes thereof align with one another. The glass is next heated under a reducing atmosphere, whereupon the fine silver halide particles are reduced to fine silver particles, thus completing the manufacture of the polarizing element.
There is also a so-called “laminate type polarizing element” (see a catalog of optical polarization control elements of Sumitomo Osaka Cement Co., Ltd.). According to this laminate type polarizing element, a laminated structure is created in which metal or semiconductor thin film layers and dielectric layers are alternately arranged one upon another on a substrate of glass or the like in a vacuum environment using vacuum deposition, sputtering or the like, with a few tens of such layers being built up in total. The substrate and the laminated structure are next sliced to a thickness of about 30 μm in a direction orthogonal to the direction of lamination. The cut surfaces created by the slicing are then smoothed by polishing, thus completing the manufacture of the polarizing element.
Furthermore, a so-called “metallic grid polarizing element” in which a metallic grid is formed on the surface of a transparent substrate has been realized (see Japanese Laid-open Patent Publication (Kokai) No. 09-304620). This polarizing element is manufactured by forming a metallic film on a transparent substrate, and then forming a metallic grid either by dry etching the metallic film using a photolithographic technique or by a lift-off method.
However, in the case of the linearly polarizing film, manufacturing the film is cheap, but because the film is manufactured by drawing a resin film, there is a problem that heat resistance and wear resistance are poor compared with an inorganic compound type polarizing element.
Moreover, in the case of the polarizing element characterized by fine silver particles having shape anisotropy being precipitated and dispersed in glass, there is a problem that, during the step in which the glass is heated under a reducing atmosphere to reduce the fine silver halide particles to fine silver particles, the fine silver halide particles may return to a spherical shape and lose their shape anisotropy or the fine silver particles may shrink in volume, the result being that incident light is scattered and hence the insertion loss of incident light increases, and thus the stability of the polarization characteristics is poor. Furthermore, there is a problem that the effects of the treatment to reduce the silver halide to metallic silver only extend to a depth of a few tens of μm from the glass surface, and hence fine silver halide particles, which do not contribute to the polarization characteristics, remain. The presence of fine silver halide particles causes a rise in the incident light insertion loss, and also makes the manufacture of the polarizing element inefficient, hampering reduction of the cost of manufacturing the polarizing element.
Moreover, in the case of the laminate type polarizing element, the manufacturing process requires a lot of time and effort, resulting in the problem of it not being possible to reduce the manufacturing cost. Moreover, adhesion at the interfaces between the metal or semiconductor film layers and the dielectric layers is very poor, and hence there is a limit on the number of layers that can be built up. Furthermore, to keep down insertion loss of incident light, it is necessary to slice the substrate and the laminated structure to a thickness of about 30 μm or less and to smooth the cut surfaces created by the slicing by polishing. Besides, the laminated structure is prone to breaking during this processing, resulting in the problem of the product yield being very poor and hence the cost of the polarizing element being very high.
Moreover, in the case of the metallic grid polarizing element, the grid spacing must be made narrower than the wavelength of the polarized light used. For example, if an optical communication wavelength of 1.55 μm is to be handled, then microprocessing to form a grid in which the grid line width and the grid spacing are of a submicron order becomes necessary, but there are limits as to what is possible with photolithographic techniques. There is a problem that if either the grid line width or the grid spacing is larger than the designated value, then the incident light will be reflected from the metallic film, resulting in the insertion loss increasing and the polarization characteristics worsening. Moreover, the dry etching selectivity ratio of the metallic film (the rate of etching of the metallic film divided by the rate of etching of the photoresist) is low, resulting in the problem that it is difficult to achieve a thick metallic film, since to etch a thick metallic film the photoresist must also be made thick.
As a result, a metallic film of a thickness sufficient for fully realizing polarization characteristics cannot be achieved. For example, in tests to double check the polarization characteristics of a grid polarizing element that uses gold as disclosed in above-mentioned Japanese Laid-open Patent Publication (Kokai) No. 09-304620, it was found that the extinction ratio was about 20 dB, which does not sufficiently meet the performance required of an optical element such as a polarizing element.
In view of the above, it is an object of the present invention to provide a polarizing element that is inexpensive and has good polarization characteristics, and a method of manufacturing this polarizing element.